Nowadays, radio systems such as wireless LAN or mobile phones have been widely used. The radio systems have been improved day by day in order to transmit more information at a faster speed.
However, some fields do not require a lot of information so much from one source. For example, the medical field (including radio calling system in the hospital), meteorologic field, disaster-prevention field, and environmental field usually require only approximately 100-bit-per-minute information. In addition, alerting for landslide requires only one-bit-per-second information.
In these fields, it is difficult to apply directly the technology about wireless LAN or mobile phones because of the problems of power consumption and costs. These fields require a radio system covering a very wide area, working by low power, and communicating by low cost (require a super-low-power long-distance communication radio system). More precisely, the following performances are required.    Communicable by approximately 50 nW output power    Communicable for a long distance (100 m for example)    Communicable one to ten bits of information per second
In addition, since a lot of information does not need to be communicated in the radio system as mentioned above, the communication speed does not need to be high-speed.
Meanwhile, identification ability about transmission media (electric wave, acoustic wave or light wave) is necessary to obtain super high sensitivity for long distance communication. The identification ability is dominated by a receiving bandwidth. A wider receiving bandwidth increases relatively natural noise power and the probability of radio interference with other communication. Therefore, an extremely narrow receiving bandwidth increases communicable distance relatively.
Moreover, the extremely narrow receiving bandwidth can reduces required power. For example, when the receiving bandwidth, approximately 20 kHz in general FM (Frequency Modulation) broadcasting, is narrowed to 1 Hz, the required power is reduced to one-to-twenty-thousand. One-to-twenty-thousand of 10 mW special-low power is 0.5 μm. When the receiving bandwidth of 10 W ham radio, which can communicate worldwide, is narrowed to 1 Hz, the required power is reduced to one-to-twenty-thousand, or to 0.5 mW.
As mentioned above, by narrowing the receiving bandwidth (for example, by narrowing to approximately 1 Hz as mentioned above) may realize a super-low-power long-distance communication radio system which is required in the mentioned fields.
However, an accuracy of the frequency of 150 MHz crystal oscillator is approximately 15 ppm; a frequency deviation of which is 3 kHz. This 3 kHz frequency deviation is 3,000 times larger than that of 1 Hz in the above example. Therefore, a carrier must be searched over the bandwidth corresponding to 1,000 channels even if every three frequency is used. Moreover, by just narrowing the receiving bandwidth, it is difficult to remove the radio interference in the conventional communication.
Accordingly, applying spread spectrum communication to narrowed receiving bandwidth, or to “narrowed occupied-bandwidth long-distance communication”, will be considered. If the spread spectrum communication can be applied, the carrier does not need to be searched and the radio interference can be removed properly.
The spread spectrum communication will be described concisely. The spread spectrum communication, the development of which was started from 1960's for military and space communication, is widely used for CDMA (Code Division Multiple Access) on mobile phones, short-distance communication by personal computers (Bluetooth), and wireless LAN (Local Area Network).
Originally, the spread spectrum communication had two aspects. The one aspect was for a long-distance communication by very weak electric-wave for military or satellite use. The other one was for multiplex communication maintaining multiple communication paths at the same frequency. The former aspect is mainly used for GPS (Global Positioning System), now employed for car navigation systems; the latter one is mainly used for the other purpose.
The spread spectrum communication reproduces an original carrier on the receiving end by multiplying a received signal by a spread signal. Before multiplying, the spread signal, applied for modulating (spreading) the original carrier on the transmitting end, is synchronized by shifting a phase of the spread signal by the time corresponding to transmission delay. To reproduce the original carrier as mentioned above is called “despread” or “demodulate”.
For despreading, the spread signal being multiplied must be shifted by the time corresponding to the transmission delay. Accordingly, in order to receive successfully on the receiving end without acknowledging the time of the transmission delay, trials of despreading (demodulating) operation is repeated while the spread code is slid little by little (for example, the shift amount which is to be provided to the spread code is increased in increments of one chip-time (the minimum time-unit of the spread code) with respect to the each trial step), and then feasibility of receiving is determined by checking the useful level of the reproduced carrier.
This synchronizing procedure as mentioned above is called “slide method”. (In addition, the carrier frequency must be known on the receiving end in the synchronizing procedure).
In the synchronizing procedure, a required time for synchronizing is roughly calculated by multiplying the period time of M-Sequence (Maximum Length Sequence) by the number of chips of M-Sequence, if M-Sequence is used as the spread code. For example, in a high-speed wireless LAN where the transmission speed is 11 Mbps and the spread code has the length corresponding to 11 chips, when the slide method is performed under the condition that the shift amount added by every trial step corresponds to one chip-time, the synchronization is established after 11 trials at the longest.
Accordingly, when one chip-time is 0.1 μs, the required time for one trial is 1.1 μs (0.1 μs×11 chips). The required time for 11 trials is 12 μs (1.1 μs×11 chips) in the slide method. In other words, the synchronization is established in such a short time.
Moreover, when the slide method is performed in GPS where the spread-code length is 1,023 chips, the required time for one trial is corresponding to 1,023 chip-times (one period time of the spread code) at the minimum. When the spread code is slid repeatedly in increments of the half of one chip-time, the synchronization is established by 2,046 slides (2,046 trials). Accordingly, the required time for synchronizing is corresponding to two-million-chip-time (1,023 chip-times×2,046). The longest required time is 2 seconds and an average required time is 1 second in GPS where the chip-time is 1 μs.
The required time for synchronizing will be considered when the spread spectrum communication, as mentioned above, is applied to “the narrowed occupied-bandwidth long-distance communication” and the slide method is performed.
The one chip-time of the spread code must be long to some extent (for example, 0.1 ms=10 kHz) when the receiving bandwidth is narrowed to approximately 1 Hz. When 1,023-chip-spread-code is applied, according to the above example of GPS, two-million-chip-time is required for synchronizing at the maximum. Accordingly, when the one chip-time of the spread code is 0.1 ms=10 kHz, the required time for synchronizing is 200 s (0.1 ms×two-million-chip-time). Therefore, an average time for synchronizing from the first receive to the first one-bit detection is 100 s; the longest time of that is 200 s.
As mentioned above, the slide method takes considerable time when the spread spectrum communication is applied to the narrowed occupied-bandwidth long-distance communication. Accordingly, an extra transmission time for synchronizing is required. However, a longer transmission time reduces electric-power-efficiency because it needs more electric power. Therefore, the super-low-power long-distance communication radio system cannot be realized by just applying the spread spectrum communication to the narrowed occupied-bandwidth long-distance communication (the first problem).
In addition, since the carrier wavelength of 150 MHz frequencies is 2 m, when a transmitter moves closer to or moves away from a receiver by 10 m/s, the receiver recognizes five-lower frequencies than the 150 MHz frequencies (150 MHz−5 Hz) or a five-higher frequencies than the 150 MHz frequencies (150 MHz+5 Hz). When the transmitter moves closer to or moves away from the receiver by 10 m/s, the chip-time of the spread code is 0.1 ms, and the spread-code length is 1,023 chips; the phase is changed 0.5 Hz, or 180 degrees, during one period-time (0.1 s).
In the above case, if an initial phase of the carrier is zero when the carrier is detected by despreading the received signal, a phase at the end part of the carrier is reversed. Therefore, the synchronization cannot be established (the second problem).
Furthermore, in the above case, establishing the synchronization takes 1,023 s at the longest when a communication rate is 1 bps and the spread-code length is 1,023 chips. Accordingly, 20 minutes from the first receiving are required before receiving data, and much time is required for synchronizing. Therefore, there is a problem for practical use (the third problem).
In addition, as a improvement of the above, an synchronization detecting method by digitizing the whole period time is known. According to the method, it seems to be able to shorten the detecting time for synchronization. However, when the carrier frequency cannot be recognized with a high degree of precision, the method causes problems such as described below.
If the carrier frequency has deviation; for example, 150 MHz±15 ppm (±2.25 kHz) (standard Xtal), one chip-time of the spread code is 0.1 ms, and the spread-code length is 1,023 chips; when the carrier frequency is changed by 5 Hz, maintaining the synchronization is difficult; when the carrier frequency is changed by 10 Hz, it can be different communication.
Accordingly, in a transmission based on phase-modulation such as the spread spectrum communication, if a phase amount of a carrier signal is not known within 0.5 period-time of the carrier over the whole 0.1 s period-time of the spread code (0.1 ms×1,023), the detection of a carrier level will fail. In short, the carrier frequency must be known precisely for synchronizing the spread code. 450 trials are required for detecting the carrier included in ±2.25 kHz in increments of 10 Hz. Therefore, when the carrier frequency has deviation, synchronizing the spread code is practically difficult (the forth problem).
Moreover, in the synchronization detecting method by digitizing the whole period time, a high-speed correlation is performed by Surface Acoustic Wave (SAW) device or others as a hardware correlation (for example, Japanese Patent Application KOKAI Publication No. 09-64787). In order to realize a long-distance communication with a low-speed communication and a low electric power, a long-time correlator is required physically. However, producing that kind of correlator is difficult (the fifth problem).
In brief, the phase must be guaranteed throughout a repeated time of the spread code in order to establish the synchronization in the conventional method; since the conventional method knows the carrier frequency or the carrier speed; can corrects the phase shifts; prepares the spread code having a proper shift amount by using that the spread-code length is short; and repeats the steps of Despread, Phase Detection, and Establishing Synchronization.
In other words, the carrier frequency must be known precisely in order to synchronize the spread code, also the spread code must be synchronized in order to know the carrier frequency.
The present invention has been invented for solving the problems as described above. An object of the present invention is to provide an equipment for spread spectrum communication which enables to realize super-low-power long-distance communication, enables to establish the synchronization at a high-speed, and enables to synchronize the spread code even if the carrier frequency is not known precisely. Another object of the present invention is to provide a synchronization establishing method for the spread spectrum communication.